multiplayer stage 2: peer identity, stable vehicle ids and a session lobby

TTrain::id() numbers cabs with a local counter, in the order each peer happens
to create them, so it cannot address a vehicle across the session. Neither was
there any way for a player to see what is available to drive.

- network/entities: PeerId (the host is peer 1, remote peers are numbered from
  2, so it is a participant rather than a special case) and NetworkEntityId,
  plus a registry that numbers the drivable vehicles of the loaded scenario in
  name order and resolves ids back to TDynamicObject
- the server owns the numbering and publishes it as VEHICLE_LIST - name, crew
  occupancy, crew capacity, AI state, availability - on every change and at
  least every 120 frames, out of band so it stays out of the replay backbuffer;
  a client binds the received ids to its own copy of the scenario by name
- SERVER_HELLO hands the joining peer its PeerId
- a multiplayer lobby listing that roster, with vehicles taken by other players
  shown as such rather than hidden; entering one goes through the existing
  entervehicle command, the vehicle is never moved

The lobby lives inside the driver mode instead of a mode of its own: the world
has to keep running while somebody is choosing (a client stops consuming the
server's frames the moment its mode is not a command processor), and the same
screen is what a player needs to change vehicles later on.

Crew occupancy is still derived from the presence of a cab instance; the real
crew model, claim handshake and AI takeover follow in stage 3.

application/driveruilayer.h

@@ -20,6 +20,7 @@ http://mozilla.org/MPL/2.0/.
 #include "widgets/trainingcard.h"
 #include "widgets/perfgraphs.h"
 #include "widgets/cameraview_extcam.h"
+#include "widgets/multiplayer_lobby.h"
 
 class driver_ui : public ui_layer {
 
@@ -40,6 +41,9 @@ public:
     // updates state of UI elements
     void
         update() override;
+    // opens or closes the multiplayer lobby
+    void
+        show_multiplayer_lobby( bool const Show );
 
 protected:
     void render_menu_contents() override;
@@ -69,4 +73,5 @@ private:
 	ui::map_panel m_mappanel;
 	ui::time_panel m_timepanel;
 	ui::cameraview_panel m_cameraviewpanel;
+	ui::multiplayer_lobby_panel m_multiplayerlobby;
 };